Switch.....on the gearbox.
Just pull the plug off and use a paper clip to bridge the terminals (reverse gear selected with ignition on) to verify that the switch itself is at fault.
Had this the other day. The flat loom terminals stretch from engine vibration: pull them of and gently squeeze them down a little with pliers so that they grip the blade on the switch firmly. Refit the wires, either way round. Hey presto! Reversing lights.
1985 Oettinger 3.2 Caravelle RHD syncro twin slider. SA Microbus bumpers, duplex winch system, ARC 7X15 period alloys
Or the switch has worn out as said. If you do replace it, you'll need to test with and without washers under it to get just the right clearance for lights to activate correctly
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